The present invention relates to controlling the diameter of beams of charged particles particularly ion beams or electron beams more particularly when such beams are used for ion writing or electron beam writing.
Ion beam writing and electron beam writing are techniques utilized in integrated circuit fabrication involving the use of focused beams of either ions or electrons which are impinged upon a target. Such beams, which have diameters in the order of 1 micron in the case of ions and sub-micron in the case of electron beams, are scanned across the target in a selected programmed fashion to produce a selected geometric pattern on the target.
The target may be a photoresist coated integrated circuit wafer in which case the pattern determines the areas in the photoresist which are exposed, as described, for example, in the article, "Electron Beam and Ion Beam Fabricated Microwave Switch," E. D. Wolf et al, I.E.E.E. Transactions on Electron Devices, Volume ED 17, No. 6., June 1970, Pages 446 - 449. The ion writing method may also involve the implantation of patterns of ions of a selected conductivity-type in a semiconductor substrate to form the active and passive regions in the integrated circuit as described in the article, "Focused Ion Beams in Microfabrication," R. L. Seliger et al, Journal Applied Physics, Volume 45, No. 3, March 1974, Pages 1416 - 1422.
In the fabrication of integrated circuits, the selected pattern which is to be formed or "written" by the beam on a wafer surface will have areas of varying lateral dimensions. In order to form such areas most effectively, it would be desirable to have the capability of varying the dimensions of the beam during the programmed writing operation. With prior art beam writing methods, beam diameter variations particularly during the programmed beam writing operation would be very cumbersome. For example, there are analyzing magnets available in ion implantation systems with rotatable pole inserts having the capability of varying the beam diameter. However, the variation of such pole pieces is conventially done manually requiring a halt in the writing operations in order to make the necessary adjustments in the pole inserts. Such an approach does not appear to be adaptable to a continuous programmed ion writing operation across the surface of a wafer.
Another approach by which the beam diameter may be varied is by changing the spacing within focusing lenses or acceleration lenses. This approach also must be done manually and is quite cumbersome requiring a halt in the ion beam or E-beam writing operations.
Still another approach of varying beam diameter involves changing the voltage levels supplied to the focusing lenses or the accelerator lenses. While this approach may be accomplished without halting the operation, any changes by this method give rise to spherical aberration which must be compensated for. Means for continuously compensating for such changes in spherical aberration are likely to be quite complex and difficult to implement in a continuous beam writing operation.